A Syrian troop buildup on the Lebanon border and comments by Syrian President Bashar al-Assad about extremist threats have stoked fears that Damascus may be planning to reassert control over its tiny neighbor.

"Without a doubt, we are very worried about there being 10,000 troops at the Lebanese border," MP and former social affairs minister Nayla Moawad told AFP.

Syria said it sent reinforcements to the border as part of a crackdown on smuggling, but the anti-Syrian camp in Lebanon has voiced concerns about its true motive.

"When there is a military buildup on one side of a border between two countries, the other country has cause for concern," said Moawad.

Saad Hariri, leader of the anti-Syrian majority in parliament, charged that the aim was to intimidate the people of Lebanon, which was under Syrian political and military domination for three decades until 2005.

Worries in Beirut deepened at comments by Assad in an interview last Sunday that northern Lebanon had turned into "a base for extremists" and "posed a threat" to his country.

"His words are a clear and direct threat to the sovereignty of Lebanon," Hariri said, arguing that the underlying message was that north Lebanon, which has been gripped by sectarian clashes, was to blame for insecurity in Syria.

Hariri's comments came after a car bombing on Monday in the northern Lebanese port city of Tripoli that killed four soldiers and three civilians, in the second bombing against the Lebanese army there in as many months.

On Saturday, a rare suicide bombing in Damascus killed 17 people, with authorities blaming a "terrorist" linked to unnamed Islamist extremists.

Syria was forced to pull its troops out of Lebanon after a 29-year deployment following the assassination of Hariri's father, ex-premier Rafik Hariri, in a massive 2005 car bombing. Syria denies involvement in the attack.

"In Lebanon, many believe that Syria has allowed extremists to infiltrate the border as they are thought to have done in Iraq," said Nadim Shehadi, a Middle East expert at Chatham House in London.

"Since its withdrawal, Syria has been keen to show that Lebanon is not viable without its control," Shehadi added.

While the United States continues to accuse Damascus of sponsoring terrorism, Damascus counters that Syria itself has become a victim of terror.

"There are Islamic extremists coming over our borders," Syrian Foreign Minister Walid Muallem has said.

Syria's support for Russia when its troops rolled into Georgia in August after Tbilisi tried to reclaim control of the Moscow-backed rebel province of South Ossetia also raised eyebrows in Lebanon.

"Let's not forget Syria's position on the Russian intervention in Georgia," said Moawad.

During a visit to Moscow in August, Assad said: "We understand Russia's military reaction and view it as a response to military provocation by Georgia."

Syria's stance raised fears in Lebanon that Damascus might make a similar move on its neighbor.

But Shehadi said: "I don't think Syria will be able to intervene militarily in Lebanon anymore."

Nonetheless, Hariri on Monday urged the international community not to allow Syria to use extremism as a pretext to meddle in Lebanese affairs.

"The Syrian leadership is searching for any reason to disrupt the course to the normalization of relations between the two countries," Hariri said.

Lebanon and Syria agreed to establish diplomatic relations for the first time at a summit in Paris in July but although Lebanese President Michel Suleiman paid a visit to Damascus last month, embassies have yet to be opened.

"If Syria had good intentions, it would have released them by now," said Shehadi, referring to Syrian activists behind bars who signed a 2006 document calling for Syrian recognition of Lebanon's independence.(AFP)